The invention relates to a lightweight support for bumpers of motor vehicles, formed by an extruded profile, with a plurality of chambers, made of a light metal. It is possible for the lightweight supports themselves to form bumpers or to support a bumper positioned in front of them or, more generally, the front part of a vehicle over the entire vehicle width. For this purpose, they are usually bent into the corresponding shape.
Such supports have to meet more and more stringent requirements in respect of economy, esthetics and safety. They are intended, on the one hand, to have the lowest weight possible and to be capable of being easily shaped as desired by the designers and, on the other hand, to meet the high strength-related requirements: bending strength, torsional strength as well as, in particular, energy absorption and deformation path in the event of a collision. In this case, for reasons of safety and of the repair costs for the various types of deformation, a reproducible transition at a certain collision speed and a certain time sequence for the deformation are sought. Moreover, connecting the support to the vehicle body via further deformable elements is intended to produce a favorable force flux.
U.S. Pat. No. 5,340,178 discloses such a support. The latter may consist of aluminum, magnesium or plastic material. Its profile is essentially rectangular, with two flattened edges and an S-shaped reinforcing rib. However, the profile, as a whole, is not particularly suitable for attachment to the vehicle body and for bending during production. The operation of bending the support into the desired shape has to take place under high internal pressure or with the use of a core because, otherwise, there is excessive deformation of the wall in the tension zone and of the S-shaped reinforcing rib.
In the aim of achieving a maximum level of energy absorption along with a minimum weight, magnesium or a magnesium alloy provides further advantages. The properties of the latter, however, differ vastly from those of aluminum or plastic. They have a smaller modulus of elasticity and a lower structural strength, on account of their hexagonal grid structure, and display different deformation behavior. In the case of deformation at room temperature (which is to be preferred for cost reasons alone), it is only possible for sliding to take place in three sliding directions in the base plane of the hexagonal basic grid. The profile described in U.S. Pat. No. 5,340,178 is not suited to the properties of magnesium alloys; the deformation in the non-supported tension zone would be too great in the case of said deformation behavior and would result in the support rupturing.
The object of the invention is to develop a support of the generic type which, along with an extremely low weight, meets all the static and dynamic requirements, is easy to bend during production and also allows for the particular properties of a magnesium alloy.
The object is achieved according to the invention wherein the profile comprises two transverse walls and side walls, which are essentially symmetrical in relation to an axis of symmetry, and two webs, which webs are arranged in the interior of the profile and parallel to the side walls.
The webs of the profile, which are symmetrical and parallel to the side walls, increase the energy absorption and support the transverse walls located in the tension and compression zones, without prematurely buckling or collapsing. This cannot be said of the S-shaped reinforcing rib according to the prior art. During the bending operation, it is then also the case that the support need not be protected by internal pressure against collapsing or subsequently calibrated. This reduces the production costs to a quite considerable extent. As a result, the profile according to the invention is also suitable for the use of a wrought magnesium alloy, and allows the advantages of this material to be utilized without it being necessary to accept the disadvantages thereof.
In a preferred configuration, the walls of the profile form essentially an isosceles trapezoid, and the webs are parallel to the side walls, which form the legs of the trapezoid. The trapezoidal shape aids the torsion of the support, in particular, if the longer of the parallel trapezoid sides is directly subjected to an impact. This improves the transmission of the forces to the vehicle body via the shorter of the parallel trapezoid sides. The inclination of the trapezoid legs and of the webs facilitates the buckling thereof, which starts at a certain impact energy, with maximum energy absorption. The webs, which are parallel to the legs of the trapezoid, support the longer of the parallel trapezoid sides under the action of the impact such that said longer side does not collapse either.
The angle of inclination of the webs in relation to the plane of symmetry of the isosceles trapezoid is preferably from 10 to 30, preferably 15 to 20, angular degrees, depending on the bending radius of the support in the plane of symmetry.
In a development of the idea of the invention for trapezoidal profiles, the webs form an accumulation of material at their transition to the shorter trapezoid side. This forms a junction which increases the buckling strength of the profile in the compression zone, increases the moment of inertia during bending by a force in the plane of symmetry, shifts the shear center further into the compression zone during torsional loading and, during extrusion, helps to even out the flow speeds.
In a particularly advantageous development of the invention in which the lightweight support is bent in the plane of symmetry, the side walls and the webs have stiffening ribs, which run transversely to the longitudinal extent of said lightweight support. These stiffening ribs further increase the strength and energy absorption of the support to an unexpectedly high extent, which has been established in tests. This allows a reduction in the wall thicknesses, and thus in the weight, to the limits of what is possible during extrusion. Thus, the wall thicknesses are preferably from 0.02 to 0.05 times the profile height along the axis of symmetry.
The support is preferably bent in the plane of symmetry, the shorter trapezoid side of the profile being on the concave side of the bent lightweight support. The profile according to the invention, however, also allows three-dimensional shaping, for example bending out of the plane of symmetry.
In certain applications, it is advantageous for the purpose of optimizing weight and deformation behavior if the webs have a lesser wall thickness than the walls forming the trapezoid and/or if at least one of the rounding radii in the interior of the profile is smaller than double the wall thickness of an adjacent wall. The latter, if the profile is regarded two-dimensionally, corresponds to less rigid clamping, with which the buckling load can be coordinated.
The invention, however, also relates to a process for producing the lightweight support according to the invention in the embodiment with the stiffening ribs. It has, surprisingly, been found that the latter are produced, in the case of the profile shape according to the invention, during bending, without a separate operation. In particular, the stiffening ribs, during bending in a die, become particularly effective and regular if the support has a trapezoidal profile. This may be explained by the oblique side walls being supported to an increasing extent on the die during the bending operation.